<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <title>Document</title>
  </head>
  <body>
    <script>
      var arr = [
        ['a', 'b', 'c'], ['d', 'e', 'f'], ['g', 'h', 'i'], ['j', 'k', 'l']
      ];
      var res = [];
      for (var i = 0; i < arr[0].length; i++) {
        res[i] = [];
        for (var j = 0; j < arr.length; j++) {
          res[i][j] = arr[j][i];	// 为二维数组赋值
        }
      }
      console.log(res);
    </script>
  </body>
</html>